1. Field of the Invention
The present invention relates to a mask pattern and to a method for forming a resist pattern using the mask pattern thereof, in particular relates to a mask pattern wherein the amplification ratio of mask CD (critical dimension: minimum dimension) dispersion at the time of transcription to a wafer by means of exposure is reduced so that the mask pattern contributes to an increase in the precision of a resist CD in the formation of a resist pattern, and relates to a method for forming a resist pattern using the mask pattern thereof.
2. Description of the Background Art
In recent years, miniaturization of resist patterns has greatly been dependant on progress in photolithographic technology, which has primarily been due to reduction in the wavelength of the exposure light. Process dimensions have generally been set at values equal to the wavelength of the exposure light, or greater, up to the 0.25 μm generation (process dimensions of 250 nm), as shown in FIG. 1.
However, at present miniaturization of process dimensions tends to be achieved, because of the high cost of exposure units, without further reduction in the wavelength of the exposure light due to progress in miniaturization of factors other than reduction in wavelength and due to super resolution technology such as scanning light exposure technology, illumination formation technology, super resolution mask technology, and the like. And, as a result, a reverse phenomenon occurs wherein process dimensions are smaller than the wavelength of exposure light (KrF excimer laser: 248 nm) from the 0.18 μm generation. A variety of technologies have been developed for the formation of resist patterns in order to deal with the further miniaturization of such process dimensions.
Japanese Unexamined Patent Publication No. HEI 3(1991)-201422 discloses a method for adjusting the difference in dimensions that occur concerning the position where the hole pattern is placed by transcribing two reticle patterns to the same position on a wafer.
Japanese Unexamined Patent Publication No. HEI 4(1992)-267537 discloses a method for preventing the transcription of a shifter edge of a phase shift mask by means of a double exposure and discloses a method for eliminating the above described resist slits by exposing and shifting a micro doped wafer followed by a second exposure.
Japanese Unexamined Patent Publication No. HEI 4(1992)-273427 and Japanese Unexamined Patent Publication No. HEI 4(1992)-355910 disclose a technique for increasing the focal point depth of an isolated pattern by means of a double exposure utilizing two masks. That is to say, according to this technique a periodic pattern including an isolated pattern is transcribed using a first mask and, then, unnecessary patterns other than the isolated pattern are exposed and eliminated using the second mask.
A variety of super resolution technologies, as described above, are utilized in photolithography for implementing process dimensions equal to, or less than, the wavelength of the exposure light and several problems occur that do not occur in photolithography for processing dimensions equal to, or greater than, the exposure light according to a prior art. The most critical problem from among these is the deterioration of MEEF (mask error enhancement factor).
The MEEF is a numeric value gained by dividing the amount of fluctuation in the resist dimensions on a wafer by the amount of fluctuation in the mask dimensions (converted value of one factor) and is utilized as an indication that shows the resultant amplification ratio wherein the dispersion in the mask dimensions corresponds to the dispersion in the resist dimensions on a wafer.
FIG. 2 is a graph showing the relationships between the actually measured values of the mask CD and the resist dimensions on a wafer. FIG. 2(a) shows the characteristics of line-type isolated patterns (target value of line width of 180 nm) and FIG. 2(b) shows the characteristics of a periodic pattern of lines and spaces (target value of line/space intervals of 180 nm). Fine lines 1, 3 in FIG. 2 indicate cases of dimension processing of the wavelength of the exposure light, or greater, according to a prior art and bold lines 2, 4 in FIG. 2 indicate cases of dimension processing of the wavelength of the exposure light, or less, according to technology developed in recent years.
The MEEF by definition corresponds to the inclination of the respective characteristics lines in FIG. 2. The MEEF is essentially 1.0 (fine line 1 of FIG. 2(a) and fine line 3 of FIG. 2(b)) in accordance with photolithography of a prior art wherein dimensions equal to, or greater than, the wavelength of the exposure light are achieved in processing. In this case the value gained by dividing the dispersion in the dimensions of the mask pattern by magnification of the scale down projection system is the dispersion in the dimensions of the resist pattern on a wafer.
However, the MEEF increases to a range of from 1.5 to 4 according to the photolithography (bold line 2 of FIG. 2(a) and bold line 4 of FIG. 2(b)) for implementing the process dimensions of the wavelength of the exposure light, or less, and, therefore, the dispersion (converted value of one factor) in the mask dimensions is amplified from 1.5 to 4 times so as to be transcribed to the resist pattern on the wafer. Here, the above described MEEF ranges widely from 1.5 to 4 because the MEEF depends on the pattern size and pitch, layout and form.
The space size (area through which exposure light passes) is considered to be constant in the line-type isolated pattern of FIG. 2(a) so that the condition of the diffracted light is stable, that is to say, fluctuation in the angle of diffraction is small and, therefore, it is understood that the value of MEEF approximates 1.0.
On the other hand, it is understood that the inclination of bold line 4 in the periodic pattern of lines/spaces of FIG. 2(b) is greater than the inclination of bold line 2 of FIG. 2(a). That is to say, it is understood that the fluctuation in the angle of diffraction corresponds to change in space size and is great resulting in deterioration in the value of MEEF because space size is small in comparison with the wavelength. It is understood that the lithography is in the condition wherein the transcription linearity has deteriorated due to the fact that the angle of diffraction becomes optically great as the space size is reduced.
As described above, the value of MEEF is a coefficient (amplification ratio of the dimensional dispersion) for determining the dimensional dispersion in the resist on the final wafer from the dimensional dispersion in the mask pattern.
In recent years, the reduction in this MEEF value has become an important issue for photolithography that implements process dimensions of the wavelength of the exposure light, or less.